1. Field of the Invention
The present invention relates to a particle-optical arrangement and to particle-optical systems, such as an electron microscopy system and an electron lithography system, comprising such particle-optical arrangement.
2. Brief Description of Related Art
In an electron microscope for inspecting an object a primary electron beam is directed to the object and secondary electrons emanating from the object are directed as a secondary electron beam to a detector.
Within the scope of the present application, the term “secondary electrons” comprises, among others:                “mirror electrons”, i.e., primary electrons which are reflected from the object and which do not fully reach the surface of the object,        “backscattering electrons”, i.e., electrons emitted from the object whose energy is substantially equal to that of the primary electrons directed to the object (elastically scattered electrons); and        “secondary electrons in the narrower sense”, i.e., those electrons emitted from the object whose kinetic energy is substantially lower than that of the primary electrons (inelastically scattered electrons).        
In electron microscopes, there is a need to separate the beam path of the primary electron beam from a beam path of the secondary electron beam in order to be able to manipulate these beams independently of each other. Thus, in an electron microscope operating with 180° reflection, i.e. an electron microscope having coinciding primary and secondary electron beams close to the object, both beams have to pass through deflecting fields of a beam splitter. Deflections of the beams provided by the beam splitter may cause a distortion, an energy dispersion and an astigmatism in the primary and secondary electron beams.
From U.S. Pat. No. 5,319,207 there is known a particle-optical system having a beam splitter providing low distortion, energy dispersion and astigmatism for a beam of charged particles passing therethrough. For obtaining such advantageous optical properties for one beam passing the beam splitter it is not sufficient to provide only one single magnetic field region in the beam path of this beam. At least three subsequent beam-manipulating magnetic field regions will have to be passed by the beam for providing the necessary compensations to obtain the advantageous optical properties.
Nonetheless, the magnetic beam splitter known from U.S. Pat. No. 5,319,207 has some disadvantages in view of obtaining a desired accuracy in adjusting the beam path through the beam splitter such that the advantageous optical properties are obtained.